It is well established that, in order to increase the hydrophilicity of polyurethane which is derived from polyester based polyols, carboxylic group(s), sulfonate groups(s), phosphate groups(s), or their alkali metal salt(s) are added to the polyurethane molecule (i.e., Kokai Sho 54-157603, Kokai sho 57-92422, kokai Sho 60-15473, EP-A-367667). However, when an acid group was added to the skeleton of polyurethane or an additive containing acid group(s) was mixed with polyurethane, then the hydrolysis of polyester polyol was accelerated in the presence of moisture in the air or in water, and this brought about the deterioration of the properties of polyurethane. As the neutralizing agent of the acid group, an alkali metal salt was used but the salt was not sufficient to prevent the deterioration of the polyurethane properties. Thus, a polyurethane with high hydrophilicity and good properties is needed.